There are known in the art various types of lifting devices which operate on the principle of a trapezoidal lifting mechanism. Such a mechanism usually incorporates a lower, essentially stationary platform, connected by parallel, spaced lifting arms to an upper, movable lifting platform. A pulling force exerted on the lifting platform causes the lifting platform to raise upward and be supported on the stationary platform by the lifting arms. In many instances, the stationary platform is provided with wheels attached thereto to facilitate moving the device from place to place.
Once the lifting platform has been raised, there are also various methods generally utilized for fixing or maintaining the lifting platform in this lifted position. Among the various techniques used are clamping devices connecting the lifting platform to other parts of the device and arrangements wherein the parallel lifting arms are locked in position by various locking arrangements.